This invention relates to electrolytic copper foil useful in the fabrication of printed circuit boards (PCBs), especially multi-layer printed circuit boards (MLBs), to a process for producing such foil, and to copper-clad laminates made with such foil. More particularly, this invention relates to such foil having an electrodeposited copper corrective/bond-enhancing layer applied to a matte surface of the foil.
Like many other materials used in high technology applications, electrodeposited copperfoil is a composite, i.e., it has a near-surface region with properties differing from those of the bulk material. In this sense of the word, the bulk of copper foil (core) serves in PCBs as a conductor of electricity. One of the outer surfaces of the foil serves as a substrate for image patterning and soldering to ensure necessary electrical connection between components, while the opposite side of the foil is responsible for permanently bonding conductor lines to the polymeric substrates. In addition, in the case of copper foil destined for fabrication of MLBs, the same side of the foil that is used for image patterning, serves also as a substrate for application of brown oxide treatment that is necessary for B-stage lamination.
A conventional process for making electrolytic copper foil consists essentially of two steps: first, electrodeposition or plating of a xe2x80x9cbasexe2x80x9d foil on a rotating drum-cathode and second, passing the foil through a xe2x80x9ctreaterxe2x80x9d machine in order to provide one of the outer surfaces of the xe2x80x9cbasexe2x80x9d or xe2x80x9crawxe2x80x9d foil with a bonding treatment suitable for bonding to a polymeric substrate. The raw foil is pale pink in color and has two distinctly different looking sidesxe2x80x94a xe2x80x9cshiny sidexe2x80x9d, the side which was plated onto the drum surface and then stripped is quite smooth while the other side, the side which was facing toward the electrolyte and the anodes, is referred to as the xe2x80x9cmattexe2x80x9d side since it has a velvety finish, due to the difference in the growth rate of differing crystal faces during electrodeposition of the xe2x80x9cbasexe2x80x9d foil. The matte side surface, at this stage has a very fine scale micro-roughness and a very specific micro-topography. Viewed under high magnification of a scanning electron microscope, it is composed of peaks and valleys. The peaks are closely packed cones or pyramids. The cones"" height, slant, packing and shape depend, as is well known, upon closely controlled independent variables of foil thickness, current density, solution composition and temperature and the type and concentration of the addition agents and the like.
A choice exists as to whether the shiny side or the matte side of the foil should be provided with the bonding treatment. Each choice has its advantages and disadvantages. Moreover, it will depend on which segment of the PCB industry the foil is destined for: printed circuit boards that are manufactured with rigid, single sided or double sided copper clad laminates or multilayer boards. Both require high quality copper foil, but while PCB manufacturers who use rigid copper clad laminates use copper foil with bonding treatment applied to the matte side of the foil, the MLBs segment of the PCB industry might prefer copper clad laminates with bonding treatment applied to the shiny side of the foil, since in this case matte side of the foil forms the outer surface of the laminate, and the xe2x80x9cnaturalxe2x80x9d micro-roughness of the matte side contributes, as will be explained, toward quality and reliability of finished MLB.
While both rigid boards and MLB circuits these days conform to the needs of miniaturization and are manufactured with copper foil conductor or track lines that are as narrow as 5 mils, or less, it is MLBs that are the fastest growing segment of the industry, since they permit achieving the highest functional density in electronic packaging. The considerations that govern the choice whether bonding treatment should be applied to the matte side of the foil (in which shiny side of the foil forms the outer surface of the laminate), or the shiny side of the foil (in which case matte side of the foil forms the outer surface of the laminate) depend on the fundamental roles the two outer surfaces of copper foil play in the fabrication of PCBs.
The side of the foil which is provided with the bonding treatment should assure the highest possible bond strength of copper foil-polymeric substrate interface. Conversely, the opposite side of the foil which forms the top surface of copper clad laminate should assure good adhesion between this surface and photo-resist. These two requirements should be balanced against each other, with the view of achieving the optimum functional quality and performance of PCB.
The basic raw material for manufacturing printed circuits is a laminate clad with copper foil, i.e., thin copper foil firmly bonded to a substrate, e.g., a polymeric, dielectric (insulating) base material. This xe2x80x9cbondingxe2x80x9d operation is accomplished in laminating plants and involves heating and cooling cycles. Sheets of copper foil are laid on sheets of xe2x80x9cprepregxe2x80x9d (e.g., glass fabric impregnated with epoxy resin). Both materials are placed in a hydraulic press with heated pressing plates and pressed together under high pressure. At elevated temperatures, the resin liquefies and is forced, by pressure, into micro-irregularities of the foil surface. This is followed by a second cycle where both materials are cooled while pressure is maintained. The resin solidifies in the irregularities of the foil surface to firmly bond materials together, making them very difficult to pull apart.
The xe2x80x9cpeel strengthxe2x80x9d between both materials, i.e., a mechanical force required to separate two bonded materials, is increased if the bonding side of the copper foil is provided with a bonding treatment. Such bonding treatment technology and processes developed by major copper foil manufacturers are well known.
High peel strength is an extremely important characteristic since the mechanical support of circuit elements, as well as the current carrying capability of PCBs, is provided by the copper foil/polymer joint. It is essential that the foil is bonded very tightly and securely to the substrate so that the adhesive joint can withstand all PCB manufacturing steps and remain constant throughout its service lifexe2x80x94without a decrease in initial adhesion strength.
Traditionally, in rigid, single sided or double sided copper clad laminates the xe2x80x9cshinyxe2x80x9d side (drum side) of the foil represents the metallic side of copper clad laminate, while the matte side (electrolyte side), responsible for permanently bonding conductor lines to the polymeric substrates. Since the highest possible bond strength (peel strength) was the most important desideratum in rigid boards technology, it was logical to combine the original micro-roughness of the matte side of the foil with the further micro-roughening effect of the electrodeposited bonding treatment.
In the case of multilayer printed circuit boards, the considerations of bondability are more complex. In the fabrication of MLBs, copper foil is laminated (bonded) to polymeric substrates twice. First, thin, double-sided copper clad laminates are produced. These laminates are then subjected to image patterning and etching away of unwanted copper to produce the desired patterns of circuitry. Several layers of double-sided boards prepared in such a manner are stacked together, with sheets of prepreg (glass reinforced polymeric resin composites) inserted between in order to dielectrically separate adjacent boards form one another. Such a stack of circuit boards and prepreg is then laminated together, in the so-called xe2x80x9cB-stage laminationxe2x80x9d, to form a monolithic multi-layer board. Later, holes are punched or drilled through the board in prearranged placed and so-called thru-hole plating of copper is used to ensure the electrical interconnection between all layers of copper-track conductor lines. Obviously, both outer surfaces of copper foil are subjected to bonding in the fabrication of MLBs and the bond strength of both surfaces is equally important.
Bonding treatment on one side of the copper foil assures bondability in the first (primary) lamination, but the top surfaces of copper circuitry have to be rendered xe2x80x9cbondablexe2x80x9d before B-stage lamination, since otherwise, the bond between copper track lines and the prepreg is not sufficient to withstand thermal shock of reflow soldering, and de-lamination takes place.
To render top surfaces of copper circuit lines adhesion prone, it is practice in the fabrication of MLBs to subject the inner layer boards, with their patterns of circuitry, to a so-called brown-oxide treatment, which changes the micro-opography of the top surfaces of the track lines to improve their bondability to the polymeric pre-preg. This brown oxide treatment is typically produced by immersing the boards in an alkaline solution of sodium chlorite, which by its oxidizing action causes the conversion of metallic copper on top surfaces of exposed copper tracks into cupric oxide CuO with a possible admixture of cuprous oxide Cu2O, depending on the type of the bath and operation conditions.
This oxide coating grows in the form of dendritic crystals, perpendicular to the surface of the copper tracks. Thus, the surface area available for bonding to polymeric substrates is increased and improved xe2x80x9cbondabilityxe2x80x9d is achieved.
As we have said earlier, this side of the copper foil which forms the top surface of copper-clad laminate is subjected to image patterning that involves the use of light sensitive materials, so-called photo-resist. The roll of a bonding treatment is to firmly anchor track lines to the polymeric substrate. After the foil is bonded to the substrate, the other side of the foil, which forms the outer surface of copper-clad laminate, is used for image patterning.
It is the practice in manufacturing printed circuit boards from copper-clad laminates to form an image of the desired printed circuit pattern on the exposed copper surface of a laminate by a photographic technique which leaves the desired pattern formed of a photo-resist material on the surface of the copper.
It will be appreciated that for the photographic imaging to be sharp and precise, the photo-resist has to spread well on the foil""s surface and adhere well to this surface.
It is a practice in manufacturing PCBs to roughen the exposed surface of the copper to achieve good resist adhesion. This roughening also removes tenacious stainproof films which foil manufacturers apply to the foil to protect it from oxidation and staining before it reaches the user. Photo-resist does not adhere to the stainproof films, which therefore have to be removed. Thus, roughening of the foil surface serves the purpose of removal of stainproof film, as well as changing the copper surface topography from smooth to micro-rough, to facilitate photo-resist adhesion which is a condition of good definition of track lines.
This roughening is performed by either mechanical means, e.g., abrasion by brushes, scrubbing with pumice, or chemical means (so-called micro-etching), which is accomplished by subjecting the copper surface of copper-clad laminates to the etching action of oxidizing mineral acids. Such acids attack the smooth surface of the foil along the copper grain boundaries, thus creating pits and pores and changing the copper surface from smooth to micro-rough.
Naturally, it is tempting to utilize the xe2x80x9cnaturalxe2x80x9d micro-roughness of the matte side of the raw foil to encourage good adhesion of photo-resist, without resorting to micro-etching or scrubbing which are costly and troublesome steps. The use of the matte side of the foil for image patterning can be accomplished by the simple expedient of providing the shiny side of the foil with the bonding treatment, and preparing copper-clad laminates in which the shiny side of the foil, with a bonding treatment electro-deposited over it, is bonded to the polymeric substrate, so that the matte side of the foil represents the top surface of the clad laminate and serves as a substrate for image patterning. In addition, such concept offers an improved quality of brown oxide treatment prior to B-stage lamination in the fabrication of MLBs.
The oxide treatment techniques used in the fabrication of MLBs are troublesome, expensive, and create their own technical problems. One, the so-called xe2x80x9cpink ringxe2x80x9d is a result in the chemical attack on copper oxide layers by the chemicals used in through-hole plating. It is customary now to engage in additional steps of brown-oxide treatment, which is a reduction of cupric oxide treatment to the metallic copper, since the bonding treatment composed of copper is immune to pink ring, as opposed to CuO which is easily dissolved in mineral acids. This reduction step further complicates brown oxide processes and renders them even more expensive.
It has been proposed that a special copper foil provided with the bonding treatment on the shiny side of the foil is better suited to fabrication of MLBs. If the bonding treatment is plated onto the drum side of the foil this results in a lower peel strength, e.g., perhaps about 8 lbs./inch than when the same treatment is plated onto the matte side of the foil, e.g., about 12 lbs./inch. Nevertheless, such peel strength is more than adequate in MLBs.
With respect to copper foil destined for use in producing MLBs, we have found that the brown oxide xe2x80x9ctreatmentxe2x80x9d which is presently applied to the shiny side of the foil and provides a quite low peel strength can advantageously by applied to the matte side of base foil, which by itself, due to its peaks and valleys topography and the resulting micro-roughness, has a considerable peel strength of about 4 lbs./inch, as opposed to the shiny side of the foil, which has substantially no peel strength at all. When this is done, very little brown oxide has to be applied to the matte side of the foil to bring the peel strength to the desired level of, e.g., 7 lbs./inch or so. This reduced amount of brown oxide is much less fragile in terms of structure, than the higher amount of brown oxide that has to be applied to the shiny side of the foil, to achieve the same peel strength. The need for reduction of cupric oxide to metallic copper can thus be eliminated, and the entire process becomes simpler and less expensive, while the quality of MLBs (particularly the dielectric properties and the resistance to delamination due to the solder shock) are improved.
Obviously, copper foil with the bonding treatment to the shiny (drum) side offers advantages in the fabrication of MLBs. That type of foil, often referred to as reverse side treated foil or drum side treated foil, is accepted in PCB industry and is a subject of several patents. These include:
U.S. Pat. No. 5,437,914 to Saida et al.
U.S. Pat. No. 5,447,619 to Wolski et al.
U.S. Pat. No. 5,545,466 to Saida et al.
U.S. Pat. No. 5,779,870 to Seip
What the improvements in printed circuits technology described in these patents do not take into account, is the lack of uniformity of the foil""s matte side surface characteristics.
Surface quality (profile) of the base foil""s matte side determines its suitability as a cladding for laminate applications destined for fine line circuitry and multi-layer printed circuit boards. The criteria of suitability depend on the quantitative evaluation of the matte side""s surface roughness. A characteristic that gives useful information about the surface is called Rz, which is the average deviation from the mean line of the five highest peaks and the five lowest valleys within the roughness sampling length. The base foil""s matte side provides the basic shape of the foil surface for embedding a substrate in the resin to promote adhesion in copper clad laminates used in manufacturing PCBs.
The surface characteristics of the matte side of the foil are, by large, the side effect of the parameters of the process employed in the fabrication of the base foil. The primary objective of the fabrication process is to satisfy the desires of the printed circuit industry by imparting a combination of physical, metallurgical, and electrical properties to the bulk of the foil. These properties are determined by the microstructure, which in turn is determined by conditions of the plating process. Typical properties of the core of the foil sought by PCB manufacturers are suitable tensile strength, yield strength, elongation, ductility, and resistance to fatigue. Many of the desired properties relate to the maximum load the material may withstand before failure, and are usually derived from stress-strain curves. Similarly, electrical conductivity is considered an important property of copper foil. All these properties of copper foil depend on the foil""s microstructure, but particularly on the microstructure of the core of the foil. This microstructure, responsible for foil""s properties, is in turn determined by electrodeposition conditions.
As a result of the fact that the main objective of the fabrication of the base foil or xe2x80x9craw foilxe2x80x9d is to impart of the body or xe2x80x9ccorexe2x80x9d of the foil, the desired physical (metallurgical) characteristics desired by printed circuit industry, the surface characteristics of the matte side of the foil (Rz, peak count) etc., vary widely from one to another manufacturer of copper foil, from one production lot to another, and from one gauge of the foil to another. We have found that this fact makes the matte side of the foil a much less dependable outer surface in copper clad laminates destined for fabrication of MLB""s.
The matte side surface represents a substrate that is responsible for photo resist adhesion and therefore precision of image patterning, optical properties of the surface responsible for high resolution and definition of circuitry, and for the anchoring of the brown oxide treatment that assures adhesion integrity of MLB""s after B stage lamination.
In that respect, copper clad laminates having the shiny side up offer at least a constancy of the surface characteristics, even if that surface requires micro-etching, or mechanical micro-roughening.
The matte side up concept, while offering a desirable micro-rough surface for subsequent processing, fails to offer constancy of the surface characteristics of the matte side, which characteristics are crucial for the actual optimizing of the fabrication stages of today""s MLB""s.
It is a general object of the present invention to provide a method of controlling the surface characteristics of the matte side of copper foil to make it more suitable for the high resolution image patterning, necessary in the fabrication of multilayer printed
A primary object of the present invention is to overcome the drawbacks of prior art foils by providing a copper foil having a constancy, from one foil to another, of matte side surface characteristics enabling improved photoresist adhesion. Another object of the invention is such a foil having a matte side with an improved peel strength compared to that of raw foil. A further object of the invention is a copper-clad laminate made with such foil, which laminate is especially suitable for use in the manufacture of MLB""s. An additional object of the invention is a process that can render the matte side of foils with different matte side microtopographies substantially alike and thus offer manufacturers of MLB a foil, which is provided with a traditional bonding treatment on the shiny side to be laminated to a polymeric substrate, and which offers a xe2x80x9ccorrectedxe2x80x9dmatte side as a substrate for subsequent MLB""s processing.
The above objects of the present invention may be achieved by an electrodeposited copper foil which comprises an electrodeposited copper base foil having a drum side with a shiny surface and an electrolyte side with a matte surface formed of micro-peaks and micro-valleys; and a copper microrough layer electrodeposited on the matte surface of the base foil, the microrough layer surface having (i) a roughness RZ different from the roughness Rz of the matte surface of the base foil and, (ii) a peak count greater than the peak count of the matte surface of the base foil.
The above copper foil can be produced by a process which comprises (a) electrodepositing a copper foil on a rotating drum cathode to produce a base copper foil having a shiny drum side and an electrolyte side with a matte surface; and (b) electrodepositing on the matte surface of the foil a copper microrough layer under electrodeposition conditions effective to electrodeposit a copper microrough layer having a surface roughness less than the roughness of matte surface and, preferably, a peak count greater than the peak count of the matte surface.
The present invention also provides a copper-clad laminate wherein the above copper foil in bonded to a polymeric substitute through a bond-enhancing copper layer electrodeposited on the drum side of the base foil.